The present invention relates to a method and apparatus for continuous measurement of liquid flow velocities and, more specifically, continuous detection of low velocities of flow for liquids.
Flow meters in use today utilize a technique based on a riser with light-emitting diodes and phototransistors. The flow measurements are carried out by reading the point in time at which a liquid surface passes a sensor, and by calculating the time difference when the liquid passes an additional sensor. The distance between the sensors and the diameter of the pipe are known, and the average velocity of flow for the time interval may thus be calculated. When the velocity of flow has been calculated, the riser must be drained before a new flow velocity may be calculated.
When there is air in the pipe, a part of the light will be reflected by the interior glass-air transition in the pipe.
When there is liquid in the pipe, the light will pass almost through without being diffused. This means that the quantity of light reaching the phototransistor for the light-emitting diode is dependent on whether there is air or liquid in the pipe. It is therefore possible to detect an absence of a liquid at a specific place in the pipe.
British patent no. 1 426 824 describes a method for improving the contrast during detection of an absence of liquid. It is described therein that, instead of measuring the passage of light in the pipe, one measures the light reflected at an approximately 105-degree angle in relation to the light source when the pipe is empty. There is greater contrast between the liquid and air for the reflecting light than for light that passes directly through the pipe. A disadvantage with the previously known devices is that it is impossible to place discrete phototransistors in close enough physical proximately to each other to achieve continuous detection of low velocities of flow. The distance between the sensor elements will result in considerable inaccuracy for low flow velocities since the position of the liquid surface cannot be determined with sufficient precision. Only average measurements can be made.
In that British patent, the use of discrete light-emitting diodes will give an inhomogeneous light, which makes it difficult to determine the position of the liquid surface beyond merely a rough, point by point detection.
Nor, in that British patent, is continuous measurement possible, since the riser must be drained between each measurement.
Nor would it be possible to carry out a flow measurement at specific time intervals since, in that British patent, one undertakes measurement of the time that it takes for the surface to move from one sensor element to another.